This invention relates to methods of producing transparent, fire resistant polycarbonate compositions and more particularly transparent, fire resistant polycarbonate compositions comprising flame retardant salts.
Plastics, and in particular polycarbonates, are increasingly being used to replace metals in a wide variety of applications, from car exteriors to aircraft interiors. The use of polycarbonate instead of metal decreases weight, improves sound dampening, and makes assembly of the device easier. Unfortunately, polycarbonates are inherently flammable, and thus require the addition of flame retardants. A variety of different materials have been used, some of which are set forth in U.S. Pat. Nos. 3,971,756 4,028,297, 4,110,299, 4,130,530, 4,303,575, 4,335,038, 4,552,911, 4,916,194, 5,218,027, and 5,508,323. The challenge is to identify economical, environmentally friendly flame retardant additives that provide the requisite flame resistance, but without compromising desirable polycarbonate properties such as strength and clarity.
Flame resistance in polycarbonate compositions may be achieved using a sulfonic acid salt such as potassium perfluorobutane sulfonate (also known as “Rimar salt”, or “KPFBS”) as disclosed, for example, in U.S. Pat. No. 3,775,367. While flame resistant, transparent polycarbonate compositions may be produced using KPFBS, optimum flame resistance is found for levels of salt that can result in haze, especially for thicker samples. The amount of flame retardant that can be added when an optically clear product is desired is thus limited. Addition of synergistic additives such as tetrabromobisphenol A to improve flame retardancy is not possible where “ECO-friendly” standard that prohibit the inclusion of bromine or chlorine are in place. Accordingly, there remains a need in the art for methods of producing polycarbonates that are not only highly flame resistant, but also transparent.